Electrode for measuring ion concentration in solutions



March 16, 1954 WHITE 2,672,441

ELECTRODE FOR MEASURING ION CONCENTRATION IN SOLUTIONS Filed Feb. 24, 1949 7 1'? l5 COMPflRMO/Y 0F ELECTRODES IN ,4 Ousunecao CoNmml/vnr'o F/LTIMTE I000 FIG. 2.

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INVENTOR'. Porn/rm Reno/Ive ROBERT J. WHITE a BY ATTORNEYS.

Patented Mar. 16, 1954 ELEGTRODE FOR' MEASURING ION CONCEN- TRATIO'N' IN SOLUTIONS Robert. J... White; Houston, tiena-l: Lead. (Comp ny-t poration of New Jersey Application February 24, 1949-, Serial No 7&085

Claims. (Cl. 204195) This invention pertains to a novel; method and means, namely; an electrode, for measuring ion concentrations in solutions. More particularlyit pertains to a potentiometri'c. method for such: determination, such method heing' applied to: the structure. of a novel electrode for that purpose.

One of. the objects: of this invention: is to; provide a rapid method: of determination oi ion concentration, more particularly of chloride ion concentrationl.

Another object is to provide such a method and means as adapted to determination of ion concentration, particularly chloride ion concentration in solutions containing: oxidizing or re.- ducing: substances.

Another object is to provide. such a method and means which. is adapted for continuous re.- cording of the values determined;

Further objects appear from the: following description, in which. will" beset forth. an illustrative embodiment. of the invention. It is to: be understood,. however, that this; invention is sue?- ceptible of various embodiments, the scope of the appended claims; without departing from the principles or spirit of the invention.

In the accompanying drawing:

Figure 1 represents a longitudinal; axial: sec;- tion of an electrode embodying this invention. The description. below indicates the preferred construction.

Figure 2 is a graph indicating, the relative accuracy of the electrode of Figure 1, a solution containing quebrachc extract as a contaminant.

Methods of measuring ion concentrations. of

solutions. by potentiometric methods involve: the

measurement of potential difierencess between the solution and electrode, which potential differences are usually very small, so that. great accuracy of measurement is: required. Moreover, the presence of impurities: inthe solution. often: causes so-called poisoning of the electrode by re.- actionwith the impurities) by which the potentials involved may be radically changed. It. has been found, however, that, by separating: the electrode from the solution by a, liquid. bath of constant concentration and contacting the solution through asuitable membrane, such poisoning effects could be reduced considerably when the membrane includes a substance. containing the ion of the solution. The theoryof. the action may be outlined as follows:

' All metals when placed. in a solution assume a potential with respect to the solution. which is dependent on the concentration of the metal-ion the solution. If we let Ag represent silver 1- E N OF F "ICE.

Tex., assignor to Na- New York, N. Y., a cormetal and Ag+ silver ion and a single bar 1 a phase boundary and. a. double bar II a liquid junction we can write a cell as follows:

Ag I At? H. relierence, electrode.

The total potential of such a cell is: made; up. 01 two parts (neglecting the liquid junction potential which canibe-made-very small). These parts are the potential between the silver metal and the solution containing silver ions which we will call E and the potential of the reference electrode which we will call Er and. which is constant. The potential E, however, depends on the concentration ofsilver ions in the following; way

where Ba is the molal electrode potential which is a constant, B. and F are: constants, T is the absolute temperature, In is the natural logarithm and [Ag+] represents. the concentration or more accurately the activity of the silver ion in contact with the silver;

When silver chloride is in contact with its saturated. solution. the. product of the. concentration of its ionsis. a. constant. called. the. solubility product. Thus we. may write [Ag+], ECl].=K.. II we should introduce into the cell above some solid silver chloride, the silver ion concentration will then depend on the concentration. of chloride ion. Such a cell would be written asfollowsz.

Ag, AgCl Cl ll reference electrode The potential. of this cell, to the left of the liquid junction is. as before but now EAg+l is equal to 33.. l and we may write or K or RT, E: EA; EAe and may be combined and written as Ellgei the molal potential of the. silver-silver chloride electrode. We then have for the potential of the complete cell a aam-icigwcm Ag, AgClIClmemb Cl- H reference electrode The potential of this cell to the left of the liquid junction is composed now of three potentials, e1, c2, and es. e1 as before is equal to Esgsr-RTZnEClr]. According to Haber the potential between a crystal lattice and its anion would be equal to AgCi Thus as the potential between the solution and the silver chloride crystal is and R e3=Tl-,T l7L[O1z"'] If we add these potentials, the potential to the left of the liquid junction is,

n% -lntclzi=m.afightin -1 The potential of the complete cell is Agc1+ rln[C1 and we again have a cell whose potential depends on the concentration of chloride on the right or outside of the membrane. In actual cells there will also be included in the righthand side of the above equation a symmetry potential due to inequalities in the two faces of the membrane.

Referring to Figure l, I designates a hollow tube of pyrex glass, originally about inches long and open at both ends. The right-hand end is closed by a membrane 2. This may be made of porous glass and impregnated with silver chloride. It is cemented or otherwise secured in the open end of the tube I so as to seal the opening. This tube is symmetrically necked down at a point 3 about 1 inch from one end. An inner tube 4 also of glass, is prepared by drawing one end to a tapered point and sealing into the point a short piece of platinum wire 5 which extends about inch beyond the point and extends about an equal distance within the tube. The tapered section of tube 4 is ground into the necked portion of tube I to form a secure ground joint seat I l. The inner tube t is removed from tube I and an electrode element 6 is formed on the end of the wire 5. This may be done by preparing a paste of 90% AgzO and 10% AgClOa, applying a small amount to the end of the wire 5 and then heating to decompose it and form a head of silver-silver chloride, or by other well known means of forming a silver or a silver-silver chloride electrode. A suitable length of rubber covered shielded cable 1 is stripped back on one end to expose a short length of the inner conductor II and the shield 8 stripped still further back so that there is no possibility of its making contact with the solder I2 which is then applied so as to form an electrical connection between the inner conductor ll and the platinum wire 5.

The end of the tube I closest to the necked down portion is prepared to receive the membrane. This preparation will take various forms according to the nature of the membrane used. In the electrode illustrated in Figure 1, the tube is ground off square and a membrane of sintered ground glass impregnated with fused silver chloride 2 is cemented by means of a high resistance cement I3 into the end of tube I in such a way that there are no leaks in the joint.

A sufificient quantity of approximately 0.1 normal solution of hydrochloric acid to about half fill the space A between the membrane 2 and the neck 3 of the tube I is introduced into this space after which tube 4, with its attached cable I, is inserted and firmly seated in ground joint I4 to seal this space, the free end of cable I extending out of the end of the tube I. Molten pitch 9 is then poured into the upper part of tube I to secure the parts in assembled relation. A metal or plastic cap I5 is slipped on the free end of cable 1 and cemented to the end of tube I with a lute or cement Ill. A suitable electrode terminal is attached to the free end of cable I.

The porous disc 2 mentioned is a disc formed by sintering a moulded disc made up of ground glass so that it is porous and coherent. The particle size of the ground glass and the degree of sintering determine the porosity. Such discs can also be made from ground Alundum and from Carborundum. In this connection it should be noted that the impregnation with silver chloride must be such that there is no final permeability and no current path except that conducted by the silver chloride itself.

The platinum wire 5 need extend only far enough inside the inner glass electrode so that it may be connected to the inner conductor of the shielded cable by means of the solder. It need not extend beyond the solder. The kind of solder which may be used is ordinary leadtin solder. The only limitation necessary on the solder is that it melt below the softening point of the glass.

The cement used between the membrane and the glass preferably has high electrical resistance compared to the resistance of the fused silver chloride membrane. The cement used between the glass and the cap is not critical and its only purpose is to secure the cap to the glass tube. The cable 8 has an inner conductor which is separated from the braided shielding of an annulus of insulation; and then this assembly is in turn covered with an exterior covering of insulation. This is very common in electronic devices.

In Figure 2, the ordinates represent the concentration (plotted on a logarithmic scale) in grams per liter of NaCl in a solution contaminated with quebracho extract, and the abscissas represent potential readings on a commercial type of vacuum-tube D. C. voltmeter, namely, a pI-I meter. Curve I represents readings taken with the electrode of Figure 1, and curve 2 readings taken with a commercial silver-silver chloride electrode presently in general use. It will be noted that curve I shows a normal change of potential with changing concentration of NaCl in this solution which contains quebracho extract, while curve 2 made with the usual silversilver chloride electrode fails to do so at concentrations of NaCl below 10 grams per liter. This is due to poisoning of the chloride by the quebracho present in the solution.

The invention having thus been described, what is claimed is:

1. An electrode for electro-determination of chloride ion concentration in a solution, comprising, a conductor connectible with potentialmeasuring means, an electrode element in contact with said conductor composed of silversilver chloride, a liquid bath containing a constant concentration of hydrochloric acid in which said electrode element is immersed, a watertight membrane of sintered glass containing silver chloride so disposed as to form a conductive path separating said bath from the solution to be measured, and means for mounting all the aforesaid elements in assembled relation to form a manipulatable electrode adapted for insertion into the solution to be measured.

2. An electrode for electro-determination of chloride ion concentration in a solution, comprising, a hollow body of inactive material having a terminal opening, a watertight membrane of coherent sintered comminuted dielectric containing a continuum of silver chloride arranged to close said opening, a liquid bath containing a constant concentration of chloride ions within said body in contact with said membrane, an electrode element of silver-silver chloride immersed in said bath, and an insulated conductor connected to said electrode element and leading out of said body.

3. An electrode for electro-determination of chloride ion concentration in a solution, comprising, a tube of inactive material having a terminal opening, a watertight membrane of sintered glass containing a continuum of silver chloride arranged to close said opening, a liquid bath containing a constant concentration of chloride ions in said tube in contact with said membrane, an electrode element of silver-silver chloride immersed in said bath, and an insulated conductor connected to said electrode element and leading out of said tube.

ROBERT J. WHITE. 

2. AN ELECTRODE FOR ELECTRO-DETERMINATION OF CHLORIDE ION CONCENTRATION IN A SOLUTION, COMPRISING, A HOLLOW BODY OF INACTIVE MATERIAL HAVING A TERMINAL OPENING, A WATERTIGHT MEMBRANE OF COHERENT SINTERED COMMINUTED DIELECTRIC CONTAINING A CONTINUUM OF SILVER CHLORIDE ARRANGED TO CLOSE SAID OPENING, A LIQUID BATH CONTAINING A CONSTANT CONCENTRATION OF CHLORIDE IONS WITHIN SAID BODY IN CONTACT WITH SAID MEMBRANE, AN ELECTRODE ELEMENT OF SILVER-SILVER CHLORIDE IMMERSED IN SAID BATH, AND AN INSULATED CONDUCTOR CONNECTED TO SAID ELECTRODE ELEMENT AND LEADING OUT OF SAID BODY. 